Ion polarizability
The polarizability of an ion consider to the capability of an applied electric field to distort the electron cloud and so induce an electric dipole moment. The very polarizable ions are large ones, particularly anions from later periods (example S2-, Br-, I-). 'Polarization' is a term frequently used loosely as meaning 'covalency' but the purely electrostatic polarizability of ions has influences that are entirely separate. In layer and chain structures anions are usually in asymmetric environments and experience a strong net electric field from neighboring ions. Polarization lowers the energy of ion in this condition, giving a stabilizing influence not possible when the coordination is symmetrical. It is noticeable that layer structures take place frequently with disulfides and dichlorides (and with heavier anions lower in similar groups), but nearly never with dioxides and difluorides: compare TiO2 and FeF2 (both rutile structure) with FeI2 and TiS2 (both CdI2 structure). Cs2O is a very rare
Instances of the anti-CdI2 structure, with adjacent layers of Cs+; the high polarizability of the Cs+ ion must be a contributing issue.
Other consequence of polarizability is the presence of vander Waals' forces between ions. They are significantly weaker than ionic forces but can have an effect on structures, particularly with large ions of high polarizability. Being short-ranged (varying with distance R as R-6) compared with Coulomb energies (R-1) they favor the maximum number of near-neighbours, irrespective of charge. It is very expected that the happening of the 8:8 CsCl structure with cesium halides (apart from CsF) is affected by this effect. Vander Waals' forces among adjacent anions are also responsible for holding together chains and layers that is another reason why ion polarizability is significant for such structures.